Field of the Invention
The present invention relates to drug-eluting medical devices; more particularly, this invention relates to support structure for medical devices. The support structure is used to support the medical device during processing of the medical device, in particular, when a coating is applied to the medical device.
Background of the Invention
Stents are often modified to provide drug delivery capabilities to further address thrombosis and restenosis. Stents may be coated with a polymeric carrier impregnated with a drug or therapeutic substance. A conventional method of coating includes applying a composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend and applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer. Depending on the application and volatility of the solvent, forced air drying may also be used to remove the solvent from the coating and arrive at a desired release rate for the impregnated therapeutic agent into the body.
It is known that some methods for coating stents do not produce an ideal, or defect-free coating over a surface of the stent where the therapeutic agent is intended to take effect. Coating defects may include non-uniform surface characteristics, such as bare spots and flaking. Coating defects can also serve as an initiation site for later-developed peeling or flaking that produces embolic debris. Rough surfaces generated by, and stagnant regions of blood flow produced by nearby flaps or packets formed by coating defects can serve as a nidus for thrombus formation. Furthermore, coating defects produce variations in the intended amount, concentration, and release rate of the drug from the stent coating, further complicating or minimizing the effectiveness of therapeutic agents.
Methods for spray coating a stent with polymer-drug dissolved in a solvent include mounting the stent on a mandrel to support and rotate the stent while it is being sprayed with the drug-polymer composition. Examples of prior mandrel designs constructed for this purpose are disclosed in U.S. Pub. No. 2007/0259100.
A support mandrel is used to hold and keep track of stents during processing. Since stents of a particular type will have different lengths or diameters, several different mandrels are needed to accommodate different stent sizes. Additionally, differently sized mandrels are needed to support different diameter stents as they are being tested or evaluated. It is desirable to have a mandrel that could be adjusted to support stents of different sizes. Many of the stent-supporting mandrels previously proposed can be adjusted to support stents of different lengths. What is needed, however, is a mandrel that can be readily adjusted to support stents of different diameters and lengths, or to adjust the amount of support needed for a stent during testing so that a single mandrel can be used to support a wider variety of stents.
In view of the foregoing, there is a need for a mandrel assembly that provides minimal contact to reduce coating defects during spraying while being adjustable to offer a variable amount of support for stents having a range of different diameters as well as lengths.